A bank (DelBank) of mouse embryonic stem (ES) cell lines is being generated by inserting by non-homologous recombination a thymidine kinase/neomycin (tk/neo) reporter cassette throughout the genome of (129 X C57BL/6J) Fl hybrid ES cells. The reporter cassette is specifically designed to allow the selection of deletion complexes in ES cells. The DelBank cell lines, each with a differently positioned reporter cassette, are then available to the research community as a resource for functional deletion mutagenesis studies. This resource will obviate the requirement of researchers to first use cumbersome gene targeting in ES cells, and it will provide researches with the capability to use specific ES cells with insertions at loci of interest to generate overlapping deletion complexes in mice. One such ES cell line will be used to generate deletion complexes in the mouse genomic region syntenic to the human Williams-Beuren disease locus. The deleted mutated mice generated will be used to assign disease phenotypes to specific genomic regions, and this will provide animal disease models for the Williams-Beuren contiguous gene syndrome.